weather satellite, artificial satellite used to gather data on a global basis for improvement of weather forecasting. Information includes cloud cover, storm location, temperature, and heat balance in the earth's atmosphere. The first weather satellites in the United States were those of the Tiros series, which began in 1960; the Nimbus series, which moved in a polar orbit, was next; the Environmental Science Services Administration (ESSA) started in 1966 and launched weather satellites; and in 1972, the Earth Resources Technology Satellite (ERTS) provided photographs to help forecasting. Other meteorological satellites include a series of Geostationary Operational Environmental Satellites (GOES), which send weather data and pictures that cover a section of the United States; China, Japan, India, and the European Space Agency (ESA) have similar craft. The National Oceanic and Atmospheric Administration's satellite series relay meteorological data to stations on the surface, including information on possible changes in various weather parameters that may signal climate change.

satellite, natural, celestial body orbiting a planet, dwarf planet, asteroid, or star of a larger size. The most familiar natural satellite is the earth's moon; thus, satellites of other planets are often referred to as moons. Within the solar system the earth's moon is the largest satellite in relation to its planet and Charon is even larger relative to the dwarf planet Pluto, although neither is the largest in actual size. The largest natural satellite, Jupiter's Ganymede, is 3,268 mi (5,262 km) in diameter, and it and Saturn's Titan are both larger than the planet Mercury. In comparison, some satellites are quite small, e.g., Deimos, the outer satellite of Mars, is c.4 mi (6 km) in diameter. Neither of the inferior planets, Mercury or Venus, has a known satellite; all of the superior planets (those whose orbits lie beyond the orbit of the earth) have at least two known satellites (Mars, 2; Jupiter, 63; Saturn, 48; Uranus, 27; Neptune, 13). A number of satellites, e.g., Phoebe of Saturn, Triton of Neptune, and most of the small outer moons of Jupiter and Uranus, have retrograde motion and may be asteroids that were captured by the planet's gravitation. The asteroid Ida has a tiny moon, Dactyl, that is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida's surface.

satellite, artificial, object constructed by humans and placed in orbit around the earth or other celestial body (see also space probe). The satellite is lifted from the earth's surface by a rocket and, once placed in orbit, maintains its motion without further rocket propulsion. The first artificial satellite, Sputnik I, was launched on Oct. 4, 1957, by the USSR; a test payload of a radio beacon and a thermometer demonstrated the feasibility of orbiting a satellite. The first U.S. satellite, Explorer I, launched on Jan. 31, 1958, returned data that was instrumental in the discovery of the Van Allen radiation belts. During the first decade of space exploration, all of the satellites were launched from either the United States or USSR. Today, there are more than three dozen launch sites in use or under construction in more than a dozen countries.

Satellite Orbits

If placed in an orbit high enough to escape the frictional effects of the earth's atmosphere, the motion of the satellite is controlled by the same laws of celestial mechanics that govern the motions of natural satellites, and it will remain in orbit indefinitely. At heights less than 200 mi (320 km) the drag produced by the atmosphere will slow the satellite down, causing it to descend into the denser portion of the atmosphere where it will burn up like a meteor. To attain orbital altitude and velocity, multistage rockets are used, with each stage falling away as its fuel is exhausted; the effect of reducing the total mass of the rocket while maintaining its thrust is to increase its speed, thus allowing it to achieve the required velocity of 5 mi per sec (8 km per sec). At this speed the rocket's forward momentum exactly balances its downward gravitational acceleration, resulting in orbit. Once above the lower atmosphere, the rocket bends to a nearly horizontal flight path, until it reaches the orbital height desired for the satellite.

Unless corrections are made, orbits are usually elliptical; perigee is the point on the orbit closest to the earth, and apogee is the point farthest from the earth. Besides this eccentricity an orbit of a satellite about the earth is characterized by its plane with respect to the earth. An equatorial orbit lies in the plane of the earth's orbit. A polar orbit lies in the plane passing through both the north and south poles. A satellite's period (the time to complete one revolution around the earth) is determined by its height above the earth; the higher the satellite, the longer the period. At a height of 200 mi (320 km), the period of a circular orbit is 90 min; at 500 mi (800 km), it increases to 100 min. At a height of 22,300 mi (36,000 km), a satellite has a period of exactly 24 hr, the time it takes the earth to rotate once on its axis; such an orbit is called geosynchronous. If the orbit is also equatorial, the satellite will remain stationary over one point on the earth's surface.

Tracking and Telemetry

Since more than 1,000 satellites are presently in orbit, identifying and maintaining contact requires precise tracking methods. Optical and radar tracking are most valuable during the launch; radio tracking is used once the satellite has achieved a stable orbit. Optical tracking uses special cameras to follow satellites illuminated either by the sun or laser beams. Radar tracking directs a pulse of microwaves at the satellite, and the reflected echo identifies both its direction and distance. Nearly all satellites carry radio transmitters that broadcast their positions to tracking antennas on the earth. In addition, the transmitters are used for telemetry, the relaying of information from the scientific instruments aboard the satellite.

Types of Satellites

Satellites can be divided into five principal types: research, communications, weather, navigational, and applications.

Research satellites measure fundamental properties of outer space, e.g., magnetic fields, the flux of cosmic rays and micrometeorites, and properties of celestial objects that are difficult or impossible to observe from the earth. Early research satellites included a series of orbiting observatories designed to study radiation from the sun, light and radio emissions from distant stars, and the earth's atmosphere. Notable research satellites have included the Hubble Space Telescope, the Compton Gamma-Ray Observatory, the Chandra X-ray Observatory, the Infrared Space Observatory, and the Solar and Heliospheric Observatory (see observatory, orbiting). Also contributing to scientific research were the experiments conducted by the astronauts and cosmonauts aboard the space stations launched by the United States (Skylab) and the Soviet Union (Salyut and Mir); in these stations researchers worked for months at a time on scientific or technical projects. The International Space Station, currently under construction, will continue this work.

Communications satellites provide a worldwide linkup of radio, telephone, and television. The first communications satellite was Echo 1; launched in 1960, it was a large metallized balloon that reflected radio signals striking it. This passive mode of operation quickly gave way to the active or repeater mode, in which complex electronic equipment aboard the satellite receives a signal from the earth, amplifies it, and transmits it to another point on the earth. Relay 1 and Telstar 1, both launched in 1962, were the first active communications satellites; Telstar 1 relayed the first live television broadcast across the Atlantic Ocean. However, satellites in the Relay and Telstar program were not in geosynchronous orbits, which is the secret to continuous communications networks. Syncom 3, launched in 1964, was the first stationary earth satellite. It was used to telecast the 1964 Olympic Games in Tokyo to the United States, the first television program to cross the Pacific Ocean. In principle, three geosynchronous satellites located symmetrically in the plane of the earth's equator can provide complete coverage of the earth's surface. In practice, many more are used in order to increase the system's message-handling capacity. The first commercial geosynchronous satellite, Intelsat 1 (better known as Early Bird), was launched by COMSAT in 1965. A network of 29 Intelsat satellites in geosynchronous orbit now provides instantaneous communications throughout the world. In addition, numerous communications satellites have been orbited by commercial organizations and individual nations for a variety of telecommunications tasks.

Weather satellites, or meteorological satellites, provide continuous, up-to-date information about large-scale atmospheric conditions such as cloud cover and temperature profiles. Tiros 1, the first such satellite, was launched in 1960; it transmitted infrared television pictures of the earth's cloud cover and was able to detect the development of hurricanes and to chart their paths. The Tiros series was followed by the Nimbus series, which carried six cameras for more detailed scanning, and the Itos series, which was able to transmit night photographs. Other weather satellites include the Geostationary Operational Environmental Satellites (GOES), which send weather data and pictures that cover a section of the United States; China, Japan, India, and the European Space Agency have orbited similar craft. Current weather satellites can transmit visible or infrared photos, focus on a narrow or wide area, and maneuver in space to obtain maximum coverage.

Navigation satellites were developed primarily to satisfy the need for a navigation system that nuclear submarines could use to update their inertial navigation system. This led the U.S. navy to establish the Transit program in 1958; the system was declared operational in 1962 after the launch of Transit 5A. Transit satellites provided a constant signal by which aircraft and ships could determine their positions with great accuracy. In 1967 civilians were able to enjoy the benefits of Transit technology. However, the Transit system had an inherent limitation. The combination of the small number of Transit satellites and their polar orbits meant there were some areas of the globe that were not continuously covered—as a result, the users had to wait until a satellite was properly positioned before they could obtain navigational information. The limitations of the Transit system spurred the next advance in satellite navigation: the availability of 24-hour worldwide positioning information. The Navigation Satellite for Time and Ranging/Global Positioning Satellite System (Navstar/GPS) consists of 24 satellites approximately 11,000 miles above the surface of the earth in six different orbital planes. The GPS has several advantages over the Transit system: It provides greater accuracy in a shorter time; users can obtain information 24 hours a day; and users are always in view of at least five satellites, which yields highly accurate location information (a direct readout of position accurate to within a few yards) including altitude. In addition, because of technological improvements, the GPS system has user equipment that is smaller and less complex. The former Soviet Union established a Navstar equivalent system known as the Global Orbiting Navigation Satellite System (GLONASS). The Russian-operated GLONASS will use the same number of satellites and orbits similar to those of Navstar when complete. Many of the handheld GPS receivers can also use the GLONASS data if equipped with the proper processing software.

Applications satellites are designed to test ways of improving satellite technology itself. Areas of concern include structure, instrumentation, controls, power supplies, and telemetry for future communications, meteorological, and navigation satellites.

Satellites also have been used for a number of military purposes, including infrared sensors that track missile launches; electronic sensors that eavesdrop on classified conversations; and optical and other sensors that aid military surveillance. Such reconnaissance satellites have subsequently proved to have civilian benefits, such as commercially available satellite photographs showing surface features and structures in great detail, and fire sensing in remote forested areas. The United States has launched several Landsat remote-imaging satellites to survey the earth's resources by means of special television cameras and radiometric scanners. Russia and other nations have also launched such satellites; the French SPOT satellite provides higher-resolution photographs of the earth.

satellite radio: see digital radio.

reconnaissance satellite, artificial satellite launched by a country to provide intelligence information on the military activities of foreign countries. There are four major types. Early-warning satellites detect enemy missile launchings. Nuclear-explosion detection satellites are designed to detect and identify nuclear explosions in space. Photo-surveillance satellites provide photographs of enemy military activities, e.g., the deployment of intercontinental ballistic missiles (ICBMs). There are two subtypes: close-look satellites provide high-resolution photographs that are returned to earth via a reentry capsule, whereas area-survey satellites provide lower-resolution photographs that are transmitted to earth via radio. Later satellites have combined these two functions. Other satellites use radar to provide images of enemy activity when there is cloud cover or it is dark. Electronic-reconnaissance (ferret) satellites pickup and record radio and radar transmissions while passing over a foreign country. The United States, Russia (before 1991, the USSR), and other nations have launched numerous reconnaissance satellites since 1960.

navigation satellite, artificial satellite designed expressly to aid the navigation of sea and air traffic. Early navigation satellites, from the Transit series launched in 1960 to the U.S. navy's Navigation Satellite System, relied on the Doppler shift. Based on the shift in the satellite's frequency, a ship at sea could accurately determine its longitude and latitude. The Global Positioning System (GPS), which uses a web of 24 Navstar satellites in 12-hour orbits, employs the more accurate triangulation method to determine position. Each satellite broadcasts time and position messages continuously. Precise to within a few yards, the GPS can also be used for nonnavigation purposes, such as surveying, tracking migrating animals, and plotting the crop yields of small sections of farmland. The former Soviet Union established a Navstar-equivalent system known as the Global Orbiting Navigation Satellite System (GLONASS). Russia's GLONASS will use the same number of satellites and orbits similar to those of Navstar when complete.

meteorological satellite: see satellite, artificial; weather satellite.

communications satellite artificial satellite that functions as part of a global radio-communications network. Echo 1, the first communications satellite, launched in 1960, was an instrumented inflatable sphere that passively reflected radio signals back to earth. Later satellites carried with them electronic devices for receiving, amplifying, and rebroadcasting signals to earth. Relay 1, launched in 1962 by the National Aeronautics and Space Administration (NASA), was the basis for Telstar 1, a commercially sponsored experimental satellite. Geosynchronous orbits (in which the satellite remains over a single spot on the earth's surface) were first used by NASA's Syncom series and Early Bird (later renamed Intelsat 1), the world's first commercial communications satellite.

In 1962, the U.S. Congress passed the Communications Satellite Act, which created the Communications Satellite Corporation (Comsat). Agencies from 17 other countries joined Comsat in 1964 in forming the International Telecommunications Satellite Consortium (Intelsat) for the purpose of establishing a global commercial communications network. Renamed the International Telecommunications Satellite Organization in 1974 and a private corporation since 2001, Intelsat now has a network of 28 satellites in geosynchronous orbits that provides instantaneous communications throughout the world. It has orbited several series of Intelsat satellites, beginning with Intelsat 1 (Early Bird) in 1965.

Inmarsat was established in 1979 to serve the maritime industry by developing satellite communications for ship management and distress and safety applications. Inmarsat was originally an intergovernmental organization called the International Maritime Satellite Organization but later changed its name to the International Mobile Satellite Organization to reflect its expansion into land, mobile, and aeronautical communications. In 1999 it became a private company as Inmarsat, and the International Mobile Satellite Organization became responsible for overseeing Inmarsat's public service obligations. Inmarsat's users now include thousands of people who live or work in remote areas without reliable terrestrial networks. Inmarsat presently has ten satellites in geosynchronous orbits.

In addition to the Intelsat and Inmarsat satellites, many others are in orbit, some managed by private companies and others by government-owned operators. These are used by individual countries, organizations, and commercial ventures for internal communications or for business or military use. A new generation of satellites, called direct-broadcast satellites, transmits directly to small domestic antennas to provide such services as cablelike television programming.

aerial and satellite photography, technology and science of taking still or moving-picture photographs from a camera mounted on a balloon, airplane, satellite, rocket, or spacecraft. In the 19th cent., photographers such as Thaddeus Lowe and George R. Lawrence took impressive pictures with cameras suspended in hot-air balloons or hung from kites, demonstrating both the scenic and military value of aerial photography. With the development of aviation, photogrammetry (the science of making measurements and maps from photographs) became an important tool. During World War I and subsequent conflicts, aerial photographs provided vital intelligence. Military aerial photography has now advanced to the point that the rank of a foot soldier can be determined from photographs taken from high-flying planes and satellites. Because of its military importance, much of the most sophisticated surveillance technology remains classified.

Aerial photography and satellite photography work in similar fashion. Course and speed are set before entering the area to be photographed, to ensure uniformity of speed and altitude. The result is an image of a narrow strip, which can be combined with overlapping images of neighboring strips to produce a panoramic view, commonly called a mosaic. Commercially available aerial and satellite photographs are capable of resolving objects of about 10 sq ft (1 sq m), which means that a satellite would be able to distinguish between a car and truck. Aerial photographs may be high oblique (including the horizon), low oblique (below the horizon), or vertical (perpendicular to the earth). Only the vertical may be accurately scaled for mapmaking purposes. Often a multilens camera is used to photograph one section vertically and the adjacent areas obliquely. The individual oblique exposures are then corrected, scaled, and joined to the vertical section to form one continuous photograph. By viewing two overlapping photographs through a stereoscope, a three-dimensional image of a region, or topographic map, can be obtained.

Images can also be produced at other wavelengths, such as microwave or infrared, by using a technique known as remote scanning, which measures variations in spectral reflectance rather than patterns of light and shadow. Remote scanning aids such disparate fields as archaeology, geology, forestry, highway construction, and land conservation. The best-known remote scanners are the Landsat series of satellites, which have mapped vegetation and geological formations on the earth's surface since 1972; the French SPOT series, first launched in 1986; Magellan, which used radar to map the planet Venus (1990); Lunar Prospector, which mapped the moon's surface composition and its magnetic and gravity fields (1998); Mars Global Surveyor, which engaged in a systematic mapping of Mars (1999); and Galileo, which returned pictures of Jupiter and its major moons (1995-2003).

Infrared Astronomical Satellite: see infrared astronomy.

Communications Satellite Corporation (Comsat), organization incorporated (1962) by an act of Congress to establish a commercial system of international communications using artificial satellites. Although government sponsored, it was financed by a public stock issue. The launching in 1965 of its first satellite, Early Bird, also known as Intelsat 1, inaugurated a transatlantic service; Asian service was established 18 months later. With more than 140 representatives of other nations, Comsat is a member of the International Telecommunications Satellite Organization, Intelsat (formerly called the International Telecommunications Satellite Consortium). Through member-company satellites and earth stations around the world, the consortium provides for international communications via telephone and television. Comsat is also the U.S. representative to the International Mobile Satellite Organization, Inmarsat (formerly called the International Maritime Satellite Organization). Established in 1979 to serve the maritime industry by developing satellite communications for ship management and distress and safety applications, Inmarsat presently has 86 member countries and has expanded into land, mobile, and aeronautical communications. See communications satellite.

Earth-orbiting system capable of receiving a signal (e.g., data, voice, TV) and relaying it back to the ground. Communications satellites have been a significant part of domestic and global communications since the 1970s. Typically they move in geosynchronous orbits about 22,300 mi (35,900 km) above the earth and operate at frequencies near 4 gigahertz (GHz) for downlinking and 6 GHz for uplinking.

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Natural object (moon) or spacecraft (artificial satellite) orbiting a larger astronomical body. Most known natural satellites orbit planets; the Earth's Moon is the most obvious example and was the only one known until the discovery of the Galilean satellites of Jupiter in 1610. All the solar system's planets except Mercury and Venus have moons, which vary greatly in size, composition (from rock to mostly ice), and activity (from cold and inert to volcanic). Some asteroids are also known to have their own moons. The first artificial satellite, Sputnik 1, was launched into orbit around Earth in 1957. Since then, thousands have been sent into orbit around Earth as well as the Moon, Venus, Mars, Jupiter, and other bodies. Artificial satellites are used for scientific research and other purposes, such as communication (see communications satellite), weather forecasting, Earth resources management, and military intelligence. Seealso Landsat.

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